PURPOSE: To enable ensurance of a given intake air amount, by a method wherein a plurality of springs the energizing forces of which are increased in rotation directions reverse to each other are mounted to a step motor to control an intake air amount, and during accident, e.g. disconnection, the rotor of a step motor is returned.

CONSTITUTION: Rotational movement of a rotor 51 of a step motor 5 is converted into linear movement by means of a lead screw 56, and the conversion regulates a gap between a valve 6 and a valve seat positioned facing the valve, and controls an air flow rate. In this case, along with rotation of the rotor 51, a gear 59, engaged with a pinion 58 and an inner gear 60 with which a reduction mechanism is formed, is revolved as it is rotated, and a reduced power is outputted to an arm 61. Along with leftward and rightward rotation of the rotor 51, springs 63 and 64 are selectively compressed through the arm 61 and holders 68 and 69. When energization to stator coils 52W55 is shut off, the rotor 51 is returned to a given position through the forces of the springs 63 and 64.